Sensor Sense: Optical Motion Sensing for Webs

Nov. 3, 2009
Noncontact motion sensors detect speed and direction of motion using a CMOS video imager.

In many printing, packaging, and general web applications, it is often necessary to know whether the web is moving and if so, in which direction. One traditional method of getting this information is to use encoders equipped with measuring wheels. The problem, however, is that there must be a mechanical coupling between the encoder wheel and the web. This coupling adds to the cost, and the added tension from the measuring wheel risks possible damage to the web.

A better method uses a noncontact optical sensor capable of detecting media motion directly. The action resembles that of a computer optical mouse dragged across a surface. The mouse detects the direction and distance of movement over the surface, and sends that information to move the cursor on the screen. But for motion sensors, it’s the web surface that moves under the stationary sensor rather than the mouse moving over a stationary surface.

The optics must be powerful enough to overcome changes to the web’s color or finish as well as any influence from ambient light. This is accomplished via a Class 2 laser LED directed at an angle 20 mm in front of the sensor. The laser light illuminates an area that is scanned by a CMOS imager array at the rate of 1,500 images/sec. The CMOS signal feeds a digital signal processor (DSP) for analysis.

The sensor detects imperfections in the web surface as patterns of light and dark points. By comparing the position of the patterns in successive images, the DSP can determine which direction the pattern moved (X or Y axis) and how far it has moved since the previous capture. The fixed rate of image capture lets the sensor calculate an average velocity of the target object as well.

Pepperl+Fuchs ( supplied information for this column.

Edited by Robert Repas

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